Phosphors, and radiation detectors and X-ray CT unit made by using the same

ABSTRACT

A novel phosphor represented by the general formula 
     
       
         (Gd 1−x Ce x ) 3 Al 5−y Ga y O 12   
       
     
     (wherein x and y are values falling in the ranges of 0.0005≦x≦0.02, and 0&lt;y&lt;5.) was provided. The phosphor has a high luminous efficiency when Al and Ga are coexistent in the Gd-system phosphor. In particular, the phosphor with the Ga content (y) in the range of 2-3 has a luminous efficiency of twice the conventional phosphor. A radiation detector using this phosphor as ceramic scintillator is capable of obtaining a high luminous output and a very small afterglow.

BACKGROUND OF THE INVENTION

The present invention relates to a rare-earth element oxide phosphorsuitable for use in a radiation detector for detecting X-rays, γ raysand the like and particularly for use in the radiation detector of anX-ray CT apparatus, a positron camera or the like. The present inventionalso relates to a radiation detector and an X-ray CT apparatus using thephosphor.

As the radiation detectors used in X-ray CT apparatuses and the likethere have conventionally been used ones combining a xenon gas chamberor BGO (bismuth germanium oxide) single crystal and a photomultipliertube or combining CsI : Tl single crystal or CdWO₄ single crystal and aphotodiode. Properties generally required of a scintillator materialused in a radiation detector include short afterglow, high luminousefficiency, high X-ray stopping power and chemical stability. Theaforementioned single crystal phosphor, however, has variations in itscharacteristics and drawbacks in any of deliquescence, cleavage,afterglow (emission after X-ray irradiation is stopped) phenomenon,luminous efficiency and the like.

In recent years, however, rare-earth-system phosphors with highradiation-to-light conversion efficiencies have been developed asscintillators and radiation detectors combining such a phosphor with aphotodiode have been put into practical use. Rare-earth phosphorsconsist of rare-earth element oxide or rare-earth element oxysulfide asbase material and an activator as luminescence component. As arare-earth element oxide phosphor, a phosphor including yttrium oxide orgadolinium oxide as base material has been proposed (Japanese PatentPublication No. 63(1988)-59436, Japanese Unexamined Patent PublicationNo.3 (1991)-50991, for example). As a rare-earth element oxysulfidephosphor, phosphors including Pr or Ce as the activator have beenproposed (Japanese Patent Publication No. 60(1985)-4856).

Although these phosphors include a phosphor having a good luminousefficiency, a phosphor having a shorter afterglow (a time required forlight to attenuate to {fraction (1/10)} after X-ray irradiation isstopped) is required depending on its application. Specifically, largeafterglow of scintillators used for detecting X-rays is particularlyproblematic in X-ray CT applications, for example, because it makesinformation-carrying signals indistinct in the time-axis direction. Verysmall afterglow is therefore required for scintillator material.However, the above-mentioned conventional rare-earth-system phosphors donot satisfy such requirement in afterglow even though they are high inluminous efficiency.

Although YAG-system phosphors (Y₃(Al,Ga)₅O₁₂) have been also known as aphosphor for electron beams (Applied Physics Letters, Jul. 15, 1967),these phosphors have low X-ray stopping power and can not be practicedin an X-ray detector.

With regard to photodetectors, the peak response wavelength of PINphotodiodes, which is currently used as photodetectors in radiationdetectors employed in X-ray CT and the like, is in the red region. Inorder to improve detection efficiency, phosphors having good wavelengthmatching with the PIN photodiodes are demanded.

An object of the present invention is therefore to provide a phosphorwith very short afterglow and high luminous efficiency that isparticularly useful as a scintillator in X-ray CT and the like. Anotherobject of the present invention is to provide a radiation detector thatis equipped with the phosphor and is high in detection efficiency.Another object of the present invention is to provide an X-ray CTapparatus that is equipped with a radiation detector with very smallafterglow and high luminous efficiency as a radiation detector and canprovide high-resolution, high-quality tomographic images.

DISCLOSURE OF THE INVENTION

In order to achieve the foregoing objects, the inventors conducted anintense study regarding rare-earth element oxide phosphors having Ce asluminous component and, discovering as a result that a phosphor havingGd₃Al_(5−y)Ga_(y)O₁₂ as base material and Ce as an activator (luminouscomponent) has high luminous efficiency and markedly low afterglow, theyarrived at present invention. The inventors also conducted an intensestudy regarding a process for manufacturing the phosphor. As a result,they found that a phosphor having markedly high luminous efficiency canbe obtained when potassium compounds are used as flux components forbaking starting materials therewith to make scintillator powder.

Specifically, the phosphor of the present invention is a phosphorrepresented by the general formula

(Gd_(1−z−x)L_(z)Ce_(x))₃Al_(5−y)Ga_(y)O₁₂

where L represents La or Y, and x, y and z are values falling in theranges of 0≦z<0.2, 0.0005≦x≦0.02, 0<y<5.

The phosphor of the present invention is a phosphor represented by theabove-described general formula and containing a very small amount ofpotassium.

The phosphor of the present invention is a phosphor represented by theabove-described general formula and obtainable by sintering thepress-molded starting materials, or by baking starting materialstogether with a flux component to make scintillator powder and sinteringthe scintillator powder after the scintillator powder is press molded.

The phosphor of the present invention includes Gd₃Al_(5−y)Ga_(y)O₁₂ asbase material and Ce as an activator (luminous component). It absorbsradiation such as X-rays and gamma rays, exhibits yellowish emission dueto Ce ion. When such a phosphor is used as a scintillator of a radiationdetector, matching with the photodiode is relatively good and a luminousoutput can be obtained that is 1.6 times or more than that of the CdWO₄currently widely used as a scintillator for X-ray CT.

The phosphor is markedly low in afterglow since it contains Ce asluminous ion and its emission attenuates to 10% by about 220 ns(nano-seconds) after X-ray irradiation is stopped and to 2×10⁻⁵ by about30 ms. Generally phosphor afterglow includes primary afterglow andsecondary afterglow (long-afterglow component). In X-ray CT, thesecondary afterglow is problematic because information-carrying signals(X-ray) become indistinct in the time-axis direction. The phosphor ismarkedly low in the secondary afterglow (afterglow after 30 ms), i.e.,2×10⁻⁵, and therefore excellent in properties suitable for scintillatorsof X-ray CT.

In the phosphor of the present invention, part of the element Gd(gadolinium) can be replaced with the element La (lanthanum) and/or theelement Y (yttrium). In this case, the phosphor remains markedly low inafterglow. However, the content of La or Y (ratio z replacing Gd) shouldbe less than 0.2, preferably less than 0.1, since as the contentincreases, the luminous efficiency and X-ray stopping power aredegraded. The luminous efficiency and X-ray stopping power can bemaximized when La or Y is not included.

By using Al (aluminum) together with Ga (gallium), high luminousefficiency can be obtained. According to the inventors' investigation,it was found that when Gd-oxide-system phosphors containing Ce asluminous component include only one of Al and Ga, that is, base materialis Gd₃Al₅O₁₂, or Gd₃Ga₅O₁₂, they do not exhibit practical amount ofemission contrary to YAG-system. However, once Al and Ga were coexistentin the phosphor, the phosphor becomes to exhibit emission and, inaddition, have markedly low afterglow. The total content of Al (5−y) andGa (y) is 5 to (Gd+L+Ce)=3 in atomic ratio, and y satisfies 0<y<5,preferably 1.7<y<3.3, more preferably 2≦y≦3. When the Al content and Gacontent are within the range of from 1.7 to 3.3 respectively, a luminousoutput that is 1.5 times or more than that of the CdWO₄ can be obtained.

Ce (Cerium) is an element that serves as an activator (luminouscomponent) in the phosphor of the present invention. The Ce content (x)in (Gd+L+Ce) for generating Ce emission is 0.0005 or greater, preferably0.001 or greater. The Ce content (x) is defined as 0.05 or less forapplications requiring high luminous output because a luminous output1.5 times that of CdWOO cannot be obtained when the Ce content (x)exceeds 0.05. Preferably, the Ce content (x) in (Gd+L+Ce) is defined as0.02 or less, more preferably 0.015 or less.

While the aforementioned elements Gd, Al, Ga and Ce are indispensableelement in the phosphor of the present invention, it may contain a verysmall amount of potassium in addition to these elements. The luminousefficiency can be further increased by addition of such a very smallamount of potassium, for example, 10 wtppm or more, preferably in therange of from 50 to 500 wtppm, more preferably-in the range of from 100to 250 wtppm. When a phosphor including potassium in the above-mentionedrange is used as a scintillator of a radiation detector, the luminousoutput twice or more than that of the CdWO₄ can be obtained.

The phosphor of the present invention may contain other elementsinevitably included therein. For example, when Gd₂O₃ is used as astarting material for manufacturing the phosphor of the presentinvention, Gd₂O₃ having purity of 99.99% may include 5 wtppm or less ofsuch impurities as Eu₂O₃, Tb₄O₇ and, therefore, the phosphor may includesuch impurities. The phosphors including such impurities are also withinthe scope of the present invention.

The phosphor of the present invention is not particularly limited withregard to crystal morphology. It may be single crystal or polycrystal.The polycrystal is preferred in view of easiness of producing and smallvariation in characteristics. The process for producing other phosphorsas single crystal reported in J. Appl. Phys., vol.42, p3049 (1971) canbe applied as the process for preparing the phosphor of the presentinvention as single crystal. The phosphor is obtained as a sinteredmaterial by hot-pressing (HP) process which adds an appropriatesintering agent to scintillator powder (starting material) and pressesit under conditions of a temperature of 1,400-1,700° C., and a pressureof about 300-1,400 atm, or by hot-isostatic pressing (HIP) process underthe same condition as that of the HP. This enables the phosphor to beobtained as a dense sintered body of high optical transmittance. Sincethe phosphor of the present invention is cubic crystal and notanisotropic in refractive index, it becomes to have high opticaltransmittance when it is made into a sintered body.

The phosphor (scintillator powder) before sintering can be prepared asfollows: mixing Gd₂O₃, Ce₂ (C₂O₄) ₃.9H₂O, Al₂O₃ and Ga₂O₃, for example,as starting material powder in a stoichiometric ratio, occasionallyadding an appropriate flux component, and conducting baking in analumina crucible at a temperature of from 1,550° C. to 1,700° C. forseveral hours.

The flux component is added in order to lower the melting temperature ofthe starting materials and expedite crystallization. As the fluxcomponent, BaF₂ used for sintering the YA-systemphosphor and potassiumcompounds such as potassium salts can be used alone or as a mixture. Thepotassium compounds such as K₂SO₄, KNO₃, K₂CO₃, K₃PO₄ are preferable.

As a result of the inventors' investigation concerning the fluxcomponents used for producing the phosphor of the present invention, itwas found that when the starting materials were baked using potassiumcompounds as the flux component, phosphors having markedly high luminousefficiency can be obtained. It is considered that the luminousefficiency is enhanced because the potassium compounds expeditecrystallization of Gd₃(Al, Ga)₅O₁₂ phase during baking and a very smallamount of the compounds is included into the crystal.

The amount of the potassium compounds used as the flux may be 0.2-1.8mol as potassium atom to lmol of the phosphor to be produced, preferably0.4-1.6 mol, more preferably 0.8-1.2 mol. With regard to compoundscontaining 2 potassium atoms in a molecule, e.g., potassium sulfate, theamount may be 0.1-0.9 mol as potassium atom to 1 mol of the phosphor tobe produced, preferably 0.2-0.8 mol, more preferably 0.4-0.6 mol.

When the amount of the flux is less or more than the aforementionedrange, deposition of crystal having another crystal phases which aredifferent from the expected crystal phase (Gd₃(Al,Ga)₂O₁₂), for exampleGdAlO₃, tend to increase. In the aforementioned range of the potassiumcompound as the flux, a very small amount (500 wtppm or less) ofpotassium is included in the produced phosphor and, as a result, thephosphor has high luminous efficiency.

The sintered body is prepared as aforementioned by using thescintillator powder baked in this way. The phosphor produced in this.manner is dense, high in optical transmittance, and small variations inits characteristics. A radiation detector of large luminous output cantherefore be obtained.

Although the phosphor of the present invention can be used inintensifying screens, fluorescent screens, scintillators and othergeneral phosphor applications, it is particularly suitable for use inX-ray CT detectors, which require high luminous output and smallafterglow.

The radiation detector of the present invention is equipped with aceramic scintillator and a photodetector for detecting scintillatoremission. The phosphor described in the foregoing is used as the ceramicscintillator. A photodiode such as a PIN photodiode is preferably usedas the photodetector. These photodiodes have high sensitivity and shortresponse. Moreover, as they have wavelength sensitivity from the visiblelight to near infrared region, they are suitable for their goodwavelength matching with the phosphor of the present invention.

The X-ray CT apparatus of the present invention is equipped with anX-ray source, an X-ray detector disposed facing the X-ray source, arevolving unit for holding the X-ray source and the X-ray detector andrevolving them about the object to be examined, and image reconstructionmeans for reconstructing a tomographic image of the object based on theintensity of the X-rays detected by the X-ray detector, which CTapparatus uses as the X-ray detector a radiation detector combining theaforesaid phosphor and a photodiode.

High-quality, high-resolution images can be obtained by utilizing thisX-ray detector because the high X-ray detection rate makes it possibleto achieve an approximate doubling of sensitivity compared with an X-rayCT apparatus using a conventional scintillator (such as CdWO₄) and alsobecause its afterglow is extremely small.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of an X-ray CT apparatusthat is an embodiment of the present invention,

FIG. 2 is a diagram showing the structure of a radiation detector (X-raydetector) that is an embodiment of the present invention,

FIG. 3 is a graph showing how detector luminous output varies with Ceconcentration in a phosphor of the present invention,

FIG. 4 is a graph showing how detector luminous output varies with Ceconcentration in a phosphor of the present invention,

FIG. 5 is a graph showing how detector luminous output varies with Gaconcentration in a phosphor of the present invention,

FIG. 6 is a graph showing how detector luminous output varies with kindsof flux components added during production of the phosphor of thepresent invention.

FIG. 7 is a graph showing how detector luminous output varies with theflux (potassium sulfate) amount, and

FIG. 8 is a graph showing how detector luminous output varies with theflux (potassium nitrate) amount.

BEST MODE FOR CARRYING OUT THE INVENTION

The X-ray CT apparatus equipped with the radiation detector of thepresent invention will now be explained with reference to an embodiment.

FIG. 1 is a schematic view of an X-ray CT apparatus of the presentinvention. The apparatus comprises a scanner gantry section 10 and animage reconstruction section 20. The scanner gantry section 10 comprisesa revolving disk 11 having an open section 14 into which the patient(the object to be examined) is conveyed, an X-ray tube 12 mounted on therevolving disk 11, a collimator 13 attached to the X-ray tube 12 forcontrolling the direction of the X-ray beam, an X-ray detector 15mounted on the revolving disk 11 to face the X-ray tube 12, a detectorcircuit 16 for converting the X-rays detected by the X-ray detector 15into a prescribed signal, and a scan control circuit 17 for controllingrevolution of the revolving disk 11 and the width of the X-ray beam.

The image reconstruction section 20 comprises an input device 21 forinputting the patient's name, date and time of the examination,examination conditions and the like, an image processing circuit 22 forprocessing measurement data S1 sent from the detector circuit 16 toeffect CT image reconstruction, image information adding section 23 foradding to the CT image produced by the image processing circuit 22 thepatient's name, date and time of the examination, examination conditionsand other information input through the input device 21, and a displaycircuit 24 for adjusting the display gain of the image-information-addedCT image signal S2 and outputting it to a display monitor 30.

X-rays are radiated from the X-ray tube 12 of the X-ray CT apparatuswith the patient resting on a table (not shown) installed in the opensection 14 of the scanner gantry section 10. The X-rays are imparteddirectivity by the collimator 13 and are detected by the X-ray detector15. By revolving the revolving disk 11 around the patient at this time,the X-rays are detected while changing the direction of the X-ray beam.In the case of a full scan, one scan is defined as one rotation (360degrees) of the revolving disk. The image of one slice is reconstructedfrom the measurement data for one scan. The tomographic image producedby the image reconstruction section 20 is displayed on the displaymonitor 30.

The X-ray detector 15 has many (e.g., 960) scintillator elements, each acombination of a scintillator and a photodiode, disposed in an arcuatearray. As shown in FIG. 2, each scintillator element has a structurecombining a scintillator 151 and a PIN photodiode 152, and the p-layerside of the PIN photodiode 152 is connected to the detector circuit 16.The whole element other than the p-layer of the PIN photodiode 152 iscovered by a shield 153 to prevent light emitted by the scintillator 151from escaping to the exterior. The shield 153 is made of a material suchas aluminum that passes X-rays and reflects light.

The scintillator 151 is a phosphor that emits light upon absorbingX-rays reaching it from the X-ray tube 12 after passing through thepatient. It consists of the phosphor of the present invention. Thescintillator 151 is higher in luminous output than conventionalscintillators. Moreover, since its emission has emission peaksstraddling the high-photosensitivity wavelength region of the PINphotodiode 152, it is photoelectrically converted by the PIN photodiode152 with high efficiency.

During the taking of tomographic images with this configuration, theX-ray tube 12 continuously emits a fan beam of X-rays as the X-ray tubeexecutes one revolution about once every 1 second to 4 seconds. Duringthis period, the X-rays passing through the object are detected, withthe detector circuit 16 side being turned ON and OFF several hundredtimes. An X-ray detector 15 with high output and short afterglow istherefore required. As the X-ray CT apparatus of the invention utilizesan X-ray detector 15 with high output and low afterglow, it can providehigh-quality CT images. Owing to the high luminous output, moreover, thesame image can be obtained with a smaller amount of X-rays, whereby theX-ray dosage received by the patient can be reduced.

Although the foregoing explanation with reference to the drawing wasmade regarding an X-ray CT apparatus using an X-ray tube, the X-raysource is not limited to an X-ray tube but can instead be a beam-typeX-ray device that effects beam scanning.

EXAMPLES Example 1

Gd₂O₃, Ce₂(C₂O₄) ₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as raw materials, andthey were mixed with a flux component, BaF₂. The mixture was packed inan alumina crucible, and, after covering the crucible, subjected tobaking at 1,600° C. for 3 hours. The baked materials were treated with2N HNO₃ aqueous solution for 1 hour to remove the flux component, washedwith water, and then dried to obtain scintillator powder.

The scintillator powder obtained in this manner was press molded and,then, the molded material was subjected to hotpressing under thecondition of 1700° C. and 500 atm. to obtain sintered body.

According to the method described above, (Gd_(l−x)Ce_(x)) ₃Al₃Ga₂O₁₂ceramic scintillators with different Ce concentrations (x) were produced(thickness of 2.5 mm). A detector was produced by using eachscintillator together with a photodiode, and placed at a distance of 15cm from an X-ray source (120 kV, 0.5 mA), and its luminous output wasmeasured.

The results are shown in FIG. 3, which are plotted with luminous outputas ordinate and Ce concentration (x) as abscissa. The luminous outputwas represented with relative values based on the luminous output of aCdWO₄ detector defined to be 1.

As clearly seen from the results shown in the figure, luminous output1.5 times or more than that of the CdWO₄ detector could be obtained inthe range of Ce concentration (x) 0.005-0.015, and 2.0 times or more inthe Ce concentration (x) of 0.002.

Example 2

Gd₂O₃,Ce₂(C₂O₄)₃.9H₂O, Al₂O₃ and Ga₂O₃were used as raw materials, andthey were mixed with a flux component, BaF₂. Then,(Gd_(1−x)Ce_(x))₃Al₂Ga₃O₁₂ ceramic scintillators with different Ceconcentrations (x) were produced (thickness of 2.5 mm) in the samemanner as in Example 1. A detector was produced by using eachscintillator together with a photodiode, and its luminous output wasmeasured in the same manner as in Example 1. The results are shown inFIG. 4, which are plotted with luminous output as ordinate and Ceconcentration (x) as abscissa.

As clearly seen from the results shown in the figure, in this Examplealso luminous output 1.5 times or more than that of the CdWO₄ detectorcould be obtained in the range of Ce concentration (x) 0.005-0.015, and1.9 times or more in the Ce concentration (x) of 0.002.

Example 3

Gd₂O₃, Ce₂(C₂O₄) ₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as raw materials, andthey were mixed with flux comrponents, BaF₂. Then,(Gd_(0.998)Ce_(0.002))₃Al_(5−y)Ga_(y)O₁₂ ceramic scintillators withdifferent Al and Ga concentrations (y) were produced (thickness of 2.5mm) in the same manner as in Example 1. A detector was produced by usingeach scintillator together with a photodiode, and its luminous outputwas measured in the same manner as in Example 1. The results are shownin FIG. 5, which are plotted with ere luminous output as ordinate and Gaconcentration (y) as abscissa.

As clearly seen from the results shown in the figure, while littleemission was observed when only one of Al and Ga was included in thescintillator, emission became to be observed both of Al and Ga wereincluded. When Ga concentration (y) was in the range of 2-3, the highestluminous output (twice that of the CdWO₄ detector) could be obtained.

Comparative Example 1

Gd₂O₃, La₂O₃, Ce₂(C₂O₄)₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as rawmaterials, and they were mixed with flux components, BaF₂. Then,(Gd_(0.898)La_(0.1)Ce_(0.002))₃Al₃Ga₂O₂ ceramic scintillator wasproduced (thickness of 2.5 mm) in the same manner as in Example 1. Adetector was produced by using the scintillator together with aphotodiode, and its luminous output was measured in the same manner asin Example 1. As a result, the luminous output was 0.8 based on that ofthe CdWO₄ detector, which was very lower than in the case that La wasnot added.

Comparative Example 2

Gd₂O₃, Y₂O₃, Ce₂(C₂O₄)₃.9H₂O, Al₂O₃ and Ga₂O₃ were use and they weremixed with flux components, BaF₂. Then, (Gd_(0.898)Y_(0.1)Ce_(0.002))₃Al₃Ga₂O₁₂ ceramic scintillator was produced (thickness of 2.5 mm) inthe same manner as in Example 1. A detector was produced by using thescintillator together with a photodiode, and its luminous output wasmeasured in the same manner as in Example 1. As a result, the luminousoutput was 1.36, based on that of CdWO₄, which was lower than in thecase where Y was not added.

Example 4

Gd₂O₃, Ce₂(C₂O₄)₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as raw materials, andthey were mixed in stoichimetric ratio. Potassium sulfate was added tothe mixture as a flux component, and the mixture was packed in analumina crucible, and, after covering the crucible, subjected to bakingat 1,600° C. for 2 hours. The amount of the flux was 0.5 mol per lmol ofthe phosphor to be obtained. The baked materials was washed thoroughlywith water to remove the flux component, and then dried to obtainscintillator powder.

The scintillator powder obtained in this manner was press molded and,then, the molded material was subjected to hot pressing under thecondition of 1500° C. and 300 atm. to obtain sintered body having acomposition of (Gd_(0.998)Ce_(0.002))₃Al₃Ga₂O₁₂.

A detector was produced by using the scintillator (thickness of 2.5 mm)together with a photodiode, and placed at a distance of 15 cm from anX-ray source (120 kV, 0.5 mA), and its luminous output was measured.

The results are shown in FIG. 6, where ordinate is the luminous outputrepresented with relative values based on the luminous output of theCdWO₄ detector defined to be 1. The luminous output of the detectorobtained in Example 3, which used a scintillator having the samecomposition (but used BaF₂ as a flux component), is also shown in thefigure.

Example 5, 6

Gd₂O₃, Ce₂(C₂O₄)₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as raw materials, andsintered bodies having a composition of (Gd_(0.998)Ce_(0.002))₃Al₃GaO₁₂were produced in the same manner as in Example 4 except that anotherflux component, potassium nitrate (Example 5) or potassium carbonate(Example 6), was used. The potassium nitrate was used in an amount of 1mol per 1 mol of the phosphor to be obtained and the potassium carbonatewas used in an amount of 0.5 mol.

Using each sintered body, a detector was produced similarly to Example 4and its luminous output was measured. The results are shown in FIG. 6.

Comparative Example 3

Gd₂O₃, Ce₂(C₂O₄) ₃.9H₂O, Al₂O₃ and Ga₂O₃ were used as raw materials, andsintered body having a composition of (Gd_(0.998)Ce_(0.002)) ₃Al₃Ga₂O₁₂,was produced in the same manner as in Example 4 except that the fluxcomponent was not used. A detector was produced similarly to Example 4using this sintered body and its luminous output was measured. Theresults are shown in FIG. 6.

As clearly seen from the results shown in the figure, high luminousoutput was obtained in the sintered body which was produced after bakingwith a flux component. Particularly, when potassium salt was used as theflux component, the luminous output was higher (about 2.2 times that ofCdWO₄) than in a case that BaF₂, a flux generally used for such asYAG-system phosphors, was used.

Example 7

Sintered bodies having a composition of (Gd_(0.998)Ce_(0.002))₃Al₃Ga₂O₁₂were produced using potassium sulfate as a flux in the same manner as inExample 4 while changing an amount of the flux component. Potassiumcontent was analyzed for the obtained sintered bodies. Using eachsintered body, a detector was produced similarly to Example 4 and itsluminous output was measured. The results are shown in FIG. 7. In thefigure, numbers indicated at each point on the graph represent thepotassium content (wtppm).

As clearly seen from the figure, the potassium content increasedapproximately linearly as more was added and when the content was in therange of 100-200 wtppm, luminous output 1.5 times or more than that ofCdWO₄ was obtained. Particularly, it showed 2 times or more with thepotassium content around 150 wtppm.

Example 8

Sintered bodies having a composition of(Gd_(0.998)Ce_(0.002))₃Al₃Ga₂O_(l2) were produced using potassiumnitrate as a flux in the same manner as that of Example 4 while changingan amount of the flux component. Potassium content was analyzed for theobtained sintered bodies. Using each sintered body, a detector wasproduced similarly to Example 4 and its luminous output was measured.The results are shown in FIG. 8. In the figure, numbers indicated ateach point on the graph represent the potassium content (wtppm).

As clearly seen from the figure, similarly to Example 7 in whichpotassium sulfate was used, when potassium nitrate was used as the fluxcomponent, high luminous output was obtained with the potassium contentof 100 wtppm or more. Particularly, it showed about 2 times or more thanthat of CdWO₄ with the potassium content around 130-150 wtppm.

Industrial Applicability

According to the present invention, there is provided a phosphor havinga composition of (Gd_(1−z−x)L_(z)Ce_(x)) ₃Al₃Ga_(y)O₁₂ and showing highluminous efficiency and very short afterglow. According to the presentinvention, there is also provided a sintered body having theaforementioned composition and high optical transmittance. A radiationdetector comprising this sintered body in combination with a siliconphotodiode advantageously shows markedly increased luminous outputcompared with a conventional detector.

What is claimed is:
 1. A phosphor obtained by press molding a startingmaterial powder and sintering the molded material, and represented bythe general formula (Gd_(1−z−x)L_(z)Ce_(x))₃Al_(5−y)Ga_(y)O₁₂ wherein Lrepresents La or Y, and x, y and z are values falling in the ranges of0.0005≦x≦0.02, 0<y<5, and 0<z<0.2.
 2. A phosphor obtained by baking astarting material powder with flux components to obtain scintillatorpowder, press molding the scintillator powder, and sintering the moldedmaterial, and represented by the general formula(Gd_(1−z−x)L_(z)Ce_(x))₃Al_(5−y)Ga_(y)O₁₂ wherein L represents La or Y,and x, y and z are values falling in the ranges of 0.0005≦x≦0.02,0<y<5,and 0≦z<0.2.
 3. The phosphor of claim 2, wherein the flux components arepotassium compounds.
 4. The phosphor of claim 3, wherein the potassiumcompound is selected from a group of potassium sulfate, potassiumnitrate and potassium carbonate. 5.The phosphor of claim 2,wherein thephosphor contains potassium in an amount of 10 wtppm or more but no morethan 500 wtppm.
 6. The phosphor of claim 3, wherein the phosphorcontains potassium in an amount of 10 wtppm or more but no more than 500wtppm.
 7. The phosphor of claim 4, wherein the phosphor containspotassium in an amount of 10 wtppm or more but no more than 500 wtppm.8. A radiation detector comprises a ceramic scintillator and aphotodetector for detecting scintillator emission, wherein said ceramicscintillator comprises a phosphor represented by the general formula(Gd_(1−z−x)L_(z)Ce_(x))₃Al_(5−y)Ga_(y)O₁₂ wherein L represents La or Y,and x, y and z are values falling in the ranges of 0.0005≦x≦0.02, 0<y<5,and 0<z<0.2.
 9. The radiation detector of claim 8, wherein thephotodetector is a photodiode.
 10. An X-ray CT apparatus comprising anX-ray source, an X-ray detector disposed facing the X-ray source, arevolving unit for holding the X-ray source and the X-ray detector andrevolving them about the object to be examined, and image reconstructionmeans for reconstructing a tomographic image of the object based on theintensity of the X-rays detected by the X-ray detector, wherein theX-ray detector is a radiation detector of claim 8.